1. Field of the Invention
The field of the invention is that of optical devices allowing the superposition of an electronic image on an image coming from an external scenery.
2. Description of the Prior Art
For a certain number of applications it is useful to blend an electronic image into an image of an external scenery. Head-up displays used principally in the aeronautic field and more recently in the automobile field will be cited. In this case, an electronic image providing flight control or navigation information is superposed on the external scenery by means of an optical combiner placed in the visual field of the pilot. Helmet displays perform the same function. The optoelectronic assembly comprising the generation of images and the combiner are, in this case, placed in or on the pilot's helmet, the visor of the helmet being able to serve as the optical combiner.
To carry out night missions, aircraft pilots also have night vision goggles. These devices are also known by the acronyms JVN, standing for “Jumelles de Vision Nocturne”, and NVG, standing for “Night Vision Goggles”. Generally NVG comprise two identical binocular bodies positioned in front of the eyes of the observer. As indicated in FIG. 1, a night vision goggles body essentially comprises 3 principal elements, which are:                a focusing objective 10;        an amplifying device 13, possessing a first, photoemissive entrance surface 11 and a second, photosensitive exit surface 12; and        an eyepiece 14.        
The objective 10 forms an image of the external scenery on the photoemissive entrance surface 11. The optical image is then converted into an electrical signal that is amplified, then converted back into an intensified optical image on the second, photosensitive exit surface 12. This intensified image is observed by means of the eyepiece 14 by the eye 15 of the observer.
When the night vision goggles are used for piloting aircraft it is necessary to provide the pilot with certain flight control information. For pilot comfort and flight safety, it is desirable that this information be superposed directly within the intensified image.
One possible solution consists in using an optical device allowing the electronic image coming from the image source to be superposed at the entrance of the objective 10. This solution has the advantage of requiring only a very low-brightness image insofar as it is amplified by the goggles' intensifier. FIG. 2 illustrates this design. The optical image superposition device comprises an image source 20, a collimation optic 21 and an optical combiner 23. In the case shown in FIG. 2, the combiner 23 is a simple beam-splitting plate. The optical device may also comprise one or more folding mirrors 22. The light rays represented by the dotted lines coming from the image source after collimation by the collimation optic and reflection at the combiner are thus superposed on the light rays coming from the external scenery, represented by the thin continuous lines.
One of the difficulties of producing this type of device is producing the optical combiner. It must introduce minimum disturbance on the external scenery. In particular, the optical transmission of the scenery through the combiner must be excellent, higher than 90%. It must also be ensured that the device has a large depth of field. This is because the external scene may include objects at distances from a few meters to several hundred meters.
U.S. Pat. No. 4,818,065 by the company Elbit discloses a first embodiment of the combiner. This is formed by two prisms assembled via a common inclined face, said common face comprising a reflecting surface that is small in size compared to that of the aperture of the NVG objective. By this means, the disturbance introduced into the field of the objective is minimized.
U.S. Pat. No. 5,502,455 by the company Honeywell discloses a second embodiment of the combiner. In this case the combiner is a simple mirror placed on the optical axis of the NVG objective. Here again the small size of the mirror allows the disturbance introduced into the field of the objective to be minimized.